Heat is often created as a byproduct of industrial processes where flowing streams of liquids, solids, or gasses that contain heat must be exhausted into the environment or otherwise removed from the process in an effort to maintain the operating temperatures of the industrial process equipment. Sometimes the industrial process can use heat exchanging devices to capture the heat and recycle it back into the process via other process streams. Other times it is not feasible to capture and recycle this heat because it is either too low in temperature or there is no readily available means to use as heat directly. This type of heat is generally referred to as “waste” heat, and is typically discharged directly into the environment through, for example, a stack, or indirectly through a cooling medium, such as water. In other settings, such heat is readily available from renewable sources of thermal energy, such as heat from the sun (which may be concentrated or otherwise manipulated) or geothermal sources. These and other thermal energy sources are intended to fall within the definition of “waste heat,” as that term is used herein.
Waste heat can be utilized by turbine generator systems which employ thermodynamic methods, such as the Rankine cycle, to convert heat into work. Supercritical CO2 thermodynamic power cycles have been proposed, which may be applied where more conventional working fluids are not well-suited. The supercritical state of the CO2 provides improved thermal coupling with multiple heat sources. For example, by using a supercritical fluid, the temperature glide of a process heat exchanger can be more readily matched. However, single-cycle, supercritical CO2 power cycles operate over a limited pressure ratio, thereby limiting the amount of temperature reduction, i.e., energy extraction, through the power conversion device (typically a turbine or positive displacement expander). The pressure ratio is limited primarily due to the high vapor pressure of the fluid at typically available condensation temperatures (e.g., ambient). As a result, the maximum output power that can be achieved from a single expansion stage is limited, and the expanded fluid retains a significant amount of potentially usable energy. While a portion of this residual energy can be recovered within the cycle by using a heat exchanger as a recuperator, and thus pre-heating the fluid between the pump and waste heat exchanger, this approach limits the amount of heat that can be extracted from the waste heat source in a single cycle.
One way to maximize the pressure ratio, and thus increase power extraction and efficiency, is to manipulate the temperature of the working fluid in the thermodynamic cycle, especially at the suction inlet of the cycle pump (or compressor). Heat exchangers, such as condensers, are typically used for this purpose, but conventional condensers are directly limited by the temperature of the cooling medium being circulated therein, which is frequently ambient air or water. On hot days, the temperature of such cooling media is heightened, which can reduce efficiency and can be especially problematic in CO2-based thermodynamic cycles or other thermodynamic cycles employing a working fluid with a critical temperature that is lower than the relatively high ambient temperature. As a result, the condenser has difficulty condensing the working fluid and cycle efficiency suffers.
Accordingly, there exists a need in the art for a system that can efficiently and effectively produce power from waste heat or other thermal sources and operates efficiently in high-ambient temperature environments.